recognize the WM even if given with a full path

Normally kWin is launched simply as "kwin" ("kwin_x11", whatever), but
in some cases it's restart command may be "/usr/bin/kwin", most notably
when KWin restarts itself after a crash. Failing to recognize this client
as the WM leads to KWin starting without any session management restoring.
CCBUG: 377756